Several fungal organisms are dimorphic, i.e., capable of existing in two forms. Such dimorphic fungi exhibit distinct morphologies in response to specific cellular signals. Typically, dimorphic fungi display either an egg-shaped, unicellular, yeast-like form, or a filamentous, mold-like form having attached and elongated cells. One example of such a dimorphism exists in the fungus Ustilago maydis in which haploid sporidia exhibit a yeast-like morphology. Such haploid sporidia may fuse to form an elongated dikaryon filamentous form if they bear distinct alleles at both the a and b compatibility loci. In contrast to the unicellular form, the filamentous form of Ustilago maydis, for example, causes corn smut.
A second well characterized example of dimorphism occurs in Candida albicans. This species of fungus exhibits a basic dimorphism between a budding yeast and a filamentous hyphal form. Several signals have been implicated in the switch between these two forms, including temperature, pH, nutrients, and exposure to serum factors. Mutants of C. albicans are known which are locked in either the yeast form or the hyphal form. Exploiting these observations for any useful purpose by classical genetic analysis is difficult, however. This is at least in part because C. albicans has only been observed as a diploid, and a sexual cycle has not been identified.
Saccharomyces cerevisiae (S. cerevisiae), also known as brewer's yeast or baker's yeast, also is a dimorphic species capable of displaying an egg-shaped yeast-like form and a filamentous mold-like form. Unfortunately, however, laboratory isolates of the fungus present a great variability in their ability to display this dimorphic characteristic. In this organism, nitrogen starvation in the presence of glucose is a natural inducer of the formation of the filamentous form, which is more appropriately termed pseudohyphae. See C. J. Gimeno et al., Cell, 68, 1078 (1992). Stimulation of a signal transduction pathway referred to as RAS2 facilitates this pseudohyphal response, i.e., the formation of a filamentous form.
S. cerevisiae pseudohyphal cells have an elongated morphology, and stay attached to each other presumably by their cell wall. Furthermore, a unipolar budding pattern occurs in which daughter cells bud, and rebud, away from their mother cell in the great majority of the cell divisions. The result is a filamentous, mold-like structure growing away from the center of the colony. Of particular note is that the pseudohyphal form of S. cerevisiae forage deeply into agar media, possibly as a result of degrading polysaccharides into energy producing monosaccharides. Thus, the pseudohyphal form of S. cerevisiae could be used in the fermentation of complex polysaccharides for the production of ethanol, for example. Unfortunately, however, the wild type S. cerevisiae only undergoes the pseudohyphal response in near-starvation conditions. Exploiting these observations could lead to significant utility in commercial fermentation applications.